Replace the shared colored-dot renderPoints for live feeds with per-feed SVG
glyphs on heading-aware divIcons:
- Aircraft: plane silhouette rotated by heading, color-coded by alt
- Trains: railcar glyph (respecting per-train iconColor), rotated when heading present
- Vessels: sailboat glyph rotated by heading, color by speed (teal/gray)
Icons are cached per (feed, color, heading) so thousands of markers reuse a
bounded set of L.divIcon instances. Rotation is applied to the inner glyph
span only (never the marker container), keeping clustering, spiderfy, and
iconAnchor intact. Missing/NaN heading falls back to an upright glyph.
Color values are sanitized against style-attribute breakout.
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